The normal method for making transmissivity measurements uses a light source on one side of the material to be tested and a detector on the other side. This emitter (light source) and detector combination is calibrated by first taking a measurement without anything between these two components. A piece of glass or other material to be tested is placed between the emitter and detector and the corresponding transmissivity can be derived from the ratio of the measured value to the calibration value. This may be a ratio of voltages or currents depending on the circuit configuration of the detector.